19. Measure of Porosity Pore size and its distribution Specific Surface Area, m 2 /g = Porosity There are three parameters used as a measure of porosity; specific surface area, specific pore volume or porosity, and pore size and its distribution. Mass of the solid, g Total surface area, m 2 Specific Pore volume, cm 3 /g Mass of the solid, g Total pore volume, cm 3 = Porosity, % = Volume of solid (including pores) Volume of pores X 100
20. Concept of Porosity: Open vs. Closed Pores Dead end (open) Closed Inter-connected (open) Passing (open) F. Rouquerol, J. Rouquerol, K. S. W. Sing, Adsorption by Powders and Porous Solids, Academic Press, 1-25, 1999 Open pores are accessible whereas closed pores are inaccessible pores. Open pores can be inter-connected, passing or dead end.
21. Size of Pores (IUPAC Standard) 2 nm 50 nm Micropores Mesopores Macropores Zeolite, Activated carbon, Metal organic framework Mesoporous silica, Activated carbon Sintered metals and ceramics Porous material are classified according to the size of pores: material with pores less than 2 nm are called micropores, materials with pores between 2 and 50 nm are called mesopores, and material with pores greater than 50 nm are macrospores Sing, K. S. W. et al. Reporting Physisorption Data for Gas/Solid Systems. Pure & Appl. Chem. 57, 603-619 (1985).
22. Shapes of Pores Conical Interstices F. Rouquerol, J. Rouquerol, K. S. W. Sing, Adsorption by Powders and Porous Solids, Academic Press, 1-25, 1999 Slits Cylindrical Spherical or Ink Bottle Pore Shapes
32. Adsorption Process Adsorption is brought by the forces acting between the solid and the molecules of the gas. These forces are of two kinds: physical (physiosorption) and chemical (chemisorption) Adsor bent - the solid where adsorption takes place Adsor bate - the gas adsorbed on the surface of solids Adsor ptive - adsorbate before being adsorbed on the surface
33. Physisorption vs Chemisorption http://www.soton.ac.uk PHYSISORPTION CHEMISORPTION WEAK, LONG RANGE BONDING Van der Waals interactions STRONG, SHORT RANGE BONDING Chemical bonding involved. NOT SURFACE SPECIFIC Physisorption takes place between all molecules on any surface providing the temperature is low enough. SURFACE SPECIFIC E.g. Chemisorption of hydrogen takes place on transition metals but not on gold or mercury. Δ H ads = 5 ….. 50 kJ mol-1 Δ H ads = 50 ….. 500 kJ mol -1 Non activated with equilibrium achieved relatively quickly. Increasing temperature always reduces surface coverage . Can be activated, in which case equilibrium can be slow and increasing temperature can favour adsorption. No surface reactions. Surface reactions may take place:- Dissociation, reconstruction, catalysis. MULTILAYER ADSORPTION BET Isotherm used to model adsorption equilibrium. MONOLAYER ADSORPTION Langmuir Isotherm is used to model adsorption equilibrium.
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35. Adsorption Process Adsor bent Adsor bate S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991 Equation of adsorption isotherm
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41. Gas Sorption: Hysteresis V a Type A Cylindrical Slits Type B Type C Type E Conical Bottle neck 1 P/P o 1 P/P o 1 P/P o Type D 1 P/P o 1 P/P o
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44. Specific Surface Area Calculation At least three data points in the relative pressure range 0.05 to 0.30 P/P o 1 V[(P o /P)-1] 0-1 0-2 0-3
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46. Pore Size Distribution V a Pore diameter, d Narrow pore size distribution Pore diameter, d Multimodal pore size distribution The distribution of pore volume with respect to pore size is called a pore size distribution. V a Broad pore size distribution Unimodal pore size distribution
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49. Pore Size: Kelvin Equation Actual radius of the pore Kelvin radius of the pore Thickness of the adsorbed layer Prior to condensation, some adsorption has taken place on the walls of the pore, r k does not represent the actual pore radius. Adsorbed layer
55. Interpretation OUTPUT Points P/P o Volume adsorbed 1 2 3 Weight of sample Pore shape Specific surface area Pore volume Pore size & distribution Results
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Editor's Notes
All solids can be classified in to two categories; porous and non-porous solids. Porous solids are those that have high surface area and high pore volume where as non-porous solids are those that have low surface area and low pore volume. In general, all solids to some extend are porous except ceramics fired at high temperatures.
The three measure of a porous solids are surface area, pore size and its distribution and pore volume.
Pores can be open or closed. Open pores are accessible where as closed pores are inaccessible. Open pores
Pores can be open or closed. Open pores are accessible where as closed pores are inaccessible. Open pores